Apparatus and method for nighttime and low visibility alignment of communicators

ABSTRACT

A method and apparatus associated therewith for acquiring and maintaining a high quality communication by communication means of the type that includes a detector and focusing means for directing encoded incoming radiation to said detector. Means are provided for determining the location of said radiation upon said detector and for generating an electrical signal responsive to said location. Said electrical signal is applied to electroluminescent means so that there is produced for the user a pattern of illumination indicating the degree of alignment of said communication means with the incoming radiation.

This is a continuation of application Ser. No. 373,075 filed Apr. 29,1982, now U.S. Pat. No. 4,603,975, which is a continuation ofapplication Ser. No. 130,637 filed Mar. 17, 1980.

TECHNICAL FIELD

The present invention relates to means for automatically aligning areceiver to an incoming laser transmission and, in particular, to meansfor achieving and maintaining good alignment between a pair ofreciprocal tracking transmitter/receivers in conditions of lowvisibility.

BACKGROUND OF THE INVENTION

The need for covert communications, military and civilian, is bothhistoric and ever-present. A communication may generally be consideredcovert if an intruder is unaware of its presence. In measuringcovertness, one may consider many parameters such as: (1) probability ofinterception (intruder's ability to receive a portion of a communicationof which he is aware); (2) jamming (intruder's as ability to interfereor limit transmission of information); and (3) spoofing (intruder'sability to interject false information without it being recognized asfalse).

In attempting to avoid detection by an intruder, a system should attemptto minimize the above-named measures. Any communication system operateswithin a 5-dimensional space. Thus, a covert system may utilize up tofive degrees of freedom to "hide" signal. The five dimensions availableare time, frequency, and the three spatial dimensions. The veiling ofthe signal is commonly accomplished by (1) concentrating all signalenergy into a small portion of the total volume of space in the hopethat an intruder will not stumble across it or (2) moving the signalenergy in some predetermined manner at a rapid rate through as much ofthe volume (of all five dimensions) as possible. In the latter instance,the intruder is required to observe the total spatial volume (ratherthan a small portion), thus incurring a reduction in receiversensitivity. Though effective in many applications, such systems exploitonly two of the above-named communication dimensions, frequency andtime. In addition, the latter dimension is exploited in only onedirection. The three spatial degrees of freedom remain unutilized forconcealment due to the finite size of the receiver's antenna. Thus,these techniques are effectively limited to about one and one halfdimensions of the five potentially available for hiding a communication.

Optical communications, on the other hand, maintain covertness throughthe concentration of signal energy into a small portion of space. Thoughnot restricted solely to communication systems, it is well known that alaser system may conveniently achieve a very small energy volumerelative to a comparable RF system. As optical systems also possess theability to take advantage of the non-spatial communication dimensions,laser communication systems exercise all the degrees of freedomavailable to RF communication systems and more. As an example of thespatial covertness which may be achieved by a laser system, the (threedimensional) volume into which energy is directed by a 10 centimeterantenna at 37.5 GHz is 64,000 times as large as the energy volumeproduced by the same size antenna (10 centimeter diameter telescope)operating at 30,000 GHz (10 micrometer wavelength). An interceptorutilizing omnidirectional receiving antennas would be assured oflocation within the radiation field of such a transmitter, allowing theconduct of a systematic frequency search with reasonable assurance ofdetection. Contrariwise, due to the nature of laser communication, notonly does the potential interceptor have to locate himself properly infrequency, time and space: he must also properly orient his antennafield-of-view to receive the transmitted energy. Thus, his searchactually has seven degrees of freedom: the five previously discussed,plus the azimuth and elevation directions of his receiving antenna'sfield-of-view. This presents a huge, multifaceted problem. (Aninterceptor utilizing an optical heterodyne receiver has a field-of-viewlimited to approximately 2.4λ÷ where λ is the wavelength and d is thediameter of the antenna; thus, a 10 centimeter receiving antenna islimited to a field-of-view on the order of 0.014 degrees).

The very desirable directionality of the optical (laser) communicationmode, which facilitates the concealment of optical transmissions inspace, gives rise to the predictable difficulties of operation found toexist when the field-of-view communications are transmitted betweentransceivers (transmitter/receivers) having unstable platforms. Thissituation has recently arisen with regard to the development oflightweight, hand-held optical communicator systems. The possibility ofdeveloping a practical line-of-sight optical communicator operable overa range of 5 to 10 miles arose with the development of the galliumarsenide (GaAs) injection laser diode. [A hand-held optical communicatorpresently available is described in "Gallium Arsenide LaserCommunicators for Hand-Held Voice or Fixed-Base Voice/Data OpticalCommunications" by Robert J. Cinzori (Procs. Sixth Conference on LaserTechnology, Department of Defense (1974))]. Optimum covertness isachieved with such a system when the solid angle of the transmission inspace is a minimum. The maintenance of a small angle of transmissionleaves little margin for the errors incurred through receiver platforminstability. The interrelationship of range and energy density imposes aphysical limit (in addition to the security considerations which dictateminimal spatial distribution of a signal) upon the solid angle of thelaser transmission. (It has been found that a communication range ofseven miles may be achieved by a present day hand-held opticalcommunicator having a transmission beamwidth of one and one-halfdegrees. This range is degraded to about one mile when the aperture isincreased to four degrees).

The maintenance of transmission between two optical communicators of thetype described above is disclosed in pending U.S. patent Ser. No.6/095,178 filed on Nov. 16, 1979 by Richard A. Dye and titled"Self-Aligning Laser Communicator Utilizing Reciprocal Tracking". Thisapplication, which is the property of the assignee herein, discloses anautomatic technique and apparatus therefor based upon the art ofreciprocal tracking. The art disclosed in the pending patent applicationessentially utilizes a quadrature (four-piece) detector and associatedcircuitry which processes the information inherent in an unequaldistribution of received energy upon such detector to continuallyreposition the reciprocal-tracking transceivers of the communicators byelectro-mechanical means. This tracking technique and apparatus iseffective provided a rough line-of-sight is maintained between thecommunicators so that incoming transmissions continue to fall with somedistribution upon the quadrature detectors of the communicators.Unfortunately, the maintenance of a rough line-of-sight may be difficultin various important applications such as shipboard platforms,battlefields, etc., and nearly impossible to establish at night and inother low-visibility situations.

Once a "line-of-sight" is established at night, it may be subject toperiodic "drop-out"--a result of the difficulty, if not impossibility,of attempting to hand-hold a communicator to within ±1° accuracy. Thus,the "shaky" base provided by the human operator will often drive thedevice to the edge of communication--a suboptimal field-of-viewalignment--whereby the automatic tracking apparatus of the Dye patentapplication must continually reposition the transceiver. This apparatusbecomes useless once the line-of-sight is lost--a distinct possibilityif such line-of-sight is on the edge of the communication.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide means forestablishing and maintaining a good line of sight between a pair ofoptical communicators, one or more of which is hand-held or has anotherwise unstable base.

Another object of the present invention is to achieve the above objectduring periods of impaired visibility, such as nighttime, therebyincreasing the utility of the hand-held communicator.

Still another object of the invention is to improve the quality ofsignal transmission and reception by enabling the receiving communicatorto continually improve the line-of-sight therebetween.

These and other objects are achieved by the method and apparatustherefor disclosed herein. A distant transmission may be located andgood quality maintained (avoiding receipt on the "edge" of the signal)by a listener utilizing a receiver having a detector according to themethod of the present invention wherein the location of the point ofincidence of said transmission upon said detector is first determinedand an electrical signal generated in response thereto. This electricalsignal is then applied to electroluminescent means comprising aplurality of electroluminescent devices arranged into a patterncorresponding to the geometry of the detector. The pattern ofillumination generated thereby gives the listener an indication of thequality of the alignment of the receiver to the transmission and therebyprovides continual guidance in the pointing of the receiver.

The determination of the location of the received energy upon thedetector is facilitated by the incorporation of a detector comprised ofa plurality of independent detector portions into the system. By the useof such a detector, as opposed to a unitary detector, each portion maycorrespond to a particular electroluminescent device of theaforementioned pattern.

Apparatus incorporating the present invention may include a plurality ofelectroluminescent devices each mounted in a segment (defined by thereticle) of the eyepiece of a sighting monocular. The sighting monocularis mounted with the receiver so that movement thereof alters thepointing direction of the receiver. Detector portions generate aplurality of electrical signals, each of which controls the luminance ofone device and indicates to the listener the present quality ofreception so that any necessary correction may be done by repointing andtransmission "drop out" thus avoided.

Other and further aspects of the present invention will become apparentduring the course of the following description and by reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, in which like numerals represent likeparts throughout:

FIG. 1 presents a conceptual view of a field communication by means of apair of optical communicators, each including the alignment apparatus ofthe present invention whereby a common line-of-sight therebetween isobtained;

FIG. 2 is a partial sectional view of an optical communicator accordingto the present invention illustrating the laser transceiver 28 and itsorientation within the outer cylinder 26;

FIG. 3 is an electromechanical schematic view of the line-of-sightdetection system of the present invention;

FIG. 4a is a side view of a communicator for use with the presentinvention illustrating the misalignment of the receiver in the verticalplane;

FIG. 4b is a planar view of the detector of the receiver of FIG. 4a,showing the movement of the blur circle thereon as a result of thealignment shown in FIG. 4a;

FIG. 4c is a view of the monocular eyepiece of the present invention;

FIG. 4d is a view of the monocular eyepiece as illuminated by theshifted blur circle of FIG. 4b;

FIG. 5a is a top view of a communicator for use with the presentinvention illustrating the misalignment of the receiver in thehorizontal plane;

FIG. 5b is a planar view of the detector of the receiver of FIG. 5ashowing the movement of the blur circle thereon as a result of thealignment shown in FIG. 5a; and

FIG. 5c is a view of the monocular eyepiece as illustrated by theshifted blur circle of FIG. 5b.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 presents a conceptual illustration of communication between afirst (speaking or transmitting) operator 10 and a second (receiving orlistening) operator 12 by means of the identical optical communicators14, 16, respectively. It is seen that the communicators 14, 16 presentrelatively lightweight, compact units which may be hand-held duringoperation. Such units have been fabricated, which are generallycylindrical in nature, having a transceiver (not shown in FIG. 1)diameter of 1.6 inches and a length of approximately 7 inches positionedinside an outer cylinder, shown in FIG. 1, having a 3.7 inch diameterand a length of 10 inches. The communicators 14, 16 utilize themonoculars 22, 24 to obtain a line of sight path 25 therebetween. Eachoperator is equipped with a headset having an earpiece 20, electricallycoupled to the receiver electronics, and a microphone 18, electricallycoupled to the transmitter electronics. Essentially, communication ofvoice is accomplished by transforming the voice of the transmittingoperator 10 into a pulsed laser transmission over the optical path orray 25. This "transformation" of voice or other data into a lasertransmission is achieved by means of the modulation of the laser energywhich travels over the path 25. The listening operator 12 receives theencoded laser transmission at the detector of his transceiver. Thetransmitted energy is converted into an electrical signal which, throughstandard transducer means, is "heard" after demodulation and decoding bythe second operator 12 over his headset. Electronics appropriate for themodulation and demodulation of such a pulsed transmission is disclosed,for a multiple signal case, in U.S. Pat. No. 4,169,213 of R. A. Dye etal for "Apparatus and Method for Ordering Independent Signals", theproperty of the assignee herein. It is essential that the operators 10,12 maintain both transceivers housed within the communicators 14, 16upon approximately coincident optical paths for the effectivetransmission and detection of the transmitted (laser) energy andassociated signal content. It is additionally imperative that the forpath 25 be held and maintained as accurately as possible as atransmission on the "edge" of the field-of-view of the receivingcommunicator is subject to data loss through scintillation, e.m.i. andrelated effects. The process by which the transmission path 25 of afirst transceiver is utilized to orient a second, receiving transceiveris known in the art as "reciprocal tracking". Pending U.S. patentapplication Ser. No. 6/095,178 of R. A. Dye for "Self-Aligning LaserCommunicator Utilizing Reciprocal Tracking" discloses means formaintaining automatic self-alignment between the transceivers of thecommunicators 14, 16. The content of that application, which is also theproperty of the assignee herein, is hereby incorporated by referenceinto this application as if restated herein.

The apparatus employed and disclosed in that application essentiallycomprises a cylindrical transceiver (transmitter-receiver) having anopposed laser transmitter and detector mounted within an outer cylinderby means of gimbaling apparatus responsive to electro-mechanicalactuators. Referring to FIG. 2, the (hollow) interior of the (outer)cylinder 26 is shown with the transceiver 28, secured by means of aholder 30, within the (outer) cylinder 26. The (inner) cylinder oroptical transceiver 28 is secured to the holder 30 by the tensioning ofa pair of fastening screws.

The transceiver 28 may be seen in FIG. 2 to be divided internally intoopposed transmitter and receiver sections 32, 34 separated by aninternal partition 36. The transmitter electronics section 32 includes aring up transformer 38 which provides power to pulser electronics 40mounted on a printed circuit board 42. The aforementioned serves todrive a compact GaAs laser diode 44 which is mounted on a heat sink 46and a holding fixture 48. A transmitter lens system 50 serves tomaintain a predetermined solid angle of optical transmission. Thisangle, the significance of which has been mentioned, supra, depends uponthe range and the degree of security desired. A conductor 52 provideselectrical connection between the transmitter and the operatormicrophone.

The optical center of the transmitter lens system 50 is intersected bythe coincident axes of the cylinder 26 and the cylindrical transceiver28. Also, coincident therewith, is the center of the IR quadrature(i.e., four equal segments) detector 54 of the receiver section 34. Thedetector 54 may be any of a number of well-known devices chosenprincipally for efficiency at the wavelength of interest. The selectionof an appropriate detector technology hinges also upon signal-to-noiseratio considerations. For example, an avalanche detector is mostdesirable in low background noise applications, due to its internalcurrent gain. Unfortunately, the current gain of an avalanche detectoracts upon both signal and noise. Thus, while signal current ismultiplied by a factor M (set by adjusting the dc bias level of thequadrature detector 54), the detector noise current, which is a functionof the background return and detector internal leakage current, ismultiplied M^(d), where d can vary from 1.5 to 2. Therefore, as themultiplication factor of the detector 54 is increased, the system S/Nratio is decreased. Generally, the limit of multiplication a system cantolerate is reached when the detector noise (with background) equals thereceiver preamplifier noise. As the field-of-view is increased, thebackground noise begins to increase proportionately with the squarethereof, and a background noise limiting situation is rapidlyapproached. A theoretical cross-over point, easily calculable, is thenreached at which a PIN detector will provide a more favorable S/N ratiothan a avalanche detector. The existence of such cross-over point ispartially a reflection of the greater quantum efficiency of the PINdetector.

The quadrature detector 54 is mounted upon a washer 56. The washer 56and the associated detector 54 are manually positioned by adjustment ofthe alignment screws 58, 60 to assure that the detector 54 remainswithin a predetermined focal spherical surface 62 when rotated in theelevation plane. The output of the detector 54 is applied to thepreamplifier 64 mounted upon the printed circuit boards 66, 68. Acomposite conductor 71 provides electrical paths to the receiverelectronics and to light emitting diodes mounted in the monoculareyepiece which provide a significant aspect of the present invention.

A spherical mirror 70 is positioned at the rear or closed end of the(outer) cylinder 26. The quadrature detector 54 is located at the focalplane 62 (approximately one-half the radius of curvature) of the mirror70 and, as mentioned above, is constrained to the movement substantiallywithin the spherical focal plane 62. This path assures that a uniform,minimum diameter, blur circle (i.e. image of laser light) will befocused upon the detector 54 at all times regardless of the relativeorientations of the cylindrical transceiver 28 and the cylinder 26.

The operation of the automatic self-alignment mechanism is described indetail in the above-referenced patent application of Dye. Essentially,the current induced in each segment of the detector 54 is processed andthen passed through differential amplifiers which determined theresultant ("push" or "pull") magnitude to be applied toelectromechanical actuators located at 45 degrees between the mechanicalaxes of a gimbaling apparatus which holds and positions the transceiver28.

FIG. 3 is an electro-mechanical schematic view of the circuitry of thepresent invention. The quadrature detector 54, divided into the labeledsegments A, B, C and D, is mounted concentrically with the crosssections of the transceiver 28 and the outer cylinder 26. The conductors72, 74, 76 and 78 transmit the currents independently generated by theincidence of portions of the incoming radiation upon the individualsegments. (The four conductors may be gathered, along with the conductorconnecting the detector to the receiver electronics, into the compositeconductor 71.) The currents therein are fed into the preamplifierelectronics 79 and then to four separate fast attack, slow decay pulseintegrators 80, 82, 84 and 86 which produce the voltages that drive theLED's 88, 90, 92, 94 mounted in the quadrants defined by the reticle 96of the monocular eyepiece 98. The communication or data signal isextracted at the receiver electronics after the combination of the four(A, B, C and D) quadrant outputs in the OR gate 100 (after thresholdingthe signals through the comparators 102, 104, 106 and 108 to ascertainwhether or not signal, as opposed to background noise, has beenreceived). The output of the OR gate 100 is then applied to thelistener's receiver demodulator electronics which may be located in theheadset electronics.

The operation and mode of use of the present invention may be observedby examining the situations illustrated in FIGS. 4(a), (b) and (c) and5(a), (b) and (c) with respect to the electro-mechanical diagram of FIG.3. In FIG. 4(a) there is illustrated a side view (similar to the view ofFIG. 2) of the transceiver 28 mounted within the outer cylinder 26.Initially, the blur circle 110, which represents the location of theincident radiation upon the surface of the quadrature detector 54 afterits reflection off the spherical mirror 70, is assumed to be centeredupon the detector (shown by the solid circle in FIG. 4b). The currentsin the four conductors 72, 74, 76 and 78 and, therefore, the voltagesapplied to the LED's from the integrators 80, 82, 84 and 86, are equal.As a result, the four LED's 88, 90, 92 and 94 are equally illuminated asshown in FIG. 4(c). (The assumption that the blur circle 110 lies at thecenter of the detector 54, implies that rays of incident radiation suchas the rays 112, 113 arrive at the communicator approximatelyperpendicular to the surface of the detector 54. As is well known in thefield of optics, such rays, after reflection from the spherical mirror70, will be focused upon the detector 54 at the center of the focalspherical plane.) The "fine tuning" tracking mechanism disclosed by Dyewill not be activated to repoint the transceiver 28 and the longitudinalaxis of the transceiver 28 will remain collinear with that of the outercylinder 26.

The dashed image of the communicator in FIG. 4a represents its positionafter an (inadvertent) amount of counterclockwise rotation has occurred.Such translation of position might occur as a result of a bouncyplatform, commonly incurred with ship-to-ship or ship-to-shorecommunications, or might result from the fact that human pointing errorsof less than ±1° are considered unrealistic. Such a small pointingerror, although often admirable and satisfactory in many applications,can quickly bring a transmission to the "edge" when a narrowfield-of-view is necessitated. The movement of the outer cylinder 26 tothe dashed position produces a rotation of the mirror 70. The result ofsuch movement is thus: rays of the incoming transmission lying above thetransceiver 28, such as the ray 112, contact the mirror 70 (afterrotation) at a point such that the distance traveled to the mirror 70 isshorter than before, as a result of which the ray 112 contacts themirror's surface at a more acute angle than previously. The reflectedray, of course, identically contacts the mirror at a more acute angle,the net result of which is a greater divergence of the reflected beam112 from the incident beam 112'. Likewise, portions of the transmissionwhich lie below the transceiver 28 travel a greater distance beforestriking the mirror 70 than before. This results in a less acute angleof incidence than before and a smaller divergence of incident ray 113from reflected ray 113'. The sum of these effects can be seen in FIG. 4bwhere there is indicated (by arrow and dashed circle) the downwardrotation of the blur circle 110 on the surface of the detector 54 thataccompanies the movement illustrated in FIG. 4a. Referring back to FIG.3, this movement of the blur circle 110 results in a diminuition of thecurrents carried along the conductors 72, 78 and an increase in thecurrents along the conductors 74, 76. Consequently, the voltages appliedto the LED's 88, 90 by the integrators 82, 84 will exceed those appliedto the LED's 92, 94 by the integrators 86, 80 respectively. Thecorresponding state of the eyepiece 98 of the monocular, as shown inFIG. 4d, thus comprises a fully illuminated pair of upper half LED's 88,90, and an unilluminated or lesser intensity pair of lower half LED's92, 94. The operator, viewing such a pattern, is informed that he is nowreceiving a transmission "on the edge" and hence directed to rotate thecommunicator's outer cylinder 26 upward (in the direction of theilluminated LED's) to restore a good field-of-view with the transmittingcommunicator. In such a manner, easily learned by the listener, anighttime communication can be kept on target so that transmission "dropout"--a very serious error in terms of nighttime corrections and/orcompensation--can be avoided.

In FIG. 5a, a top view of a communicator according to the pending patentapplication of Dye, the outer cylinder 26 is seen to have becomemisaligned with respect to the incoming transmission as the result of anunintended clockwise rotation or torque. Once again optical effects,analogous to those described above, act upon the incident radiation tocause the ray 114 to the right of the transceiver 28 to diverge uponreflection from the mirror 70 less than before while the ray 115,arriving on the left hand side of the transceiver 28, diverges, uponreflection from the mirror, a greater amount than before. The net resultof these effects is indicated in FIG. 5b by the arrow and dashed circleindicating the rightward movement of the blur circle 110 on the surfaceof the detector 54. As a result of this movement, the currents carriedby the conductors 72, 74 increase while those carried by the conductors76, 78 decrease. The relative current values result in a greater voltageoutput from the integrators 80, 82 than from the integrators 84, 86 anda corresponding increase in the illumination afforded by the LED' s 88,94 relative to that afforded by the LED's 90, 92. The pattern therebysuperimposed upon the eyepiece of the monocular is shown in FIG. 5c toconsist of a brightly illuminated pair of LED's 88, 94 to the left handside of the reticle 96 and a dimly (if at all) illuminated pair of LED's90, 92 to the right hand side of the reticle 96. Once again, thelistener is notified that his hand movements and the like have moved thecommunicator off center and he is now at the "edge" and close totransmission drop out. To correct therefor, he must rotate the front ofthe communicator toward the illuminated half of the eyepiece (that is,counterclockwise) until the full complement of eyepiece LED's isilluminated as before.

Thus, it is seen that the present invention affords the user (listener)of a communication system of the nature described an easily understoodand implemented apparatus and method for maintaining a "good"line-of-sight so that maximum transmission/reception quality can bemaintained. The apparatus of the present invention, which employs, interalia, illuminative elements, solves critical nighttime communicationproblems by allowing the user a convenient, non-distracting means foranalyzing the varying quality of communication and may be employed, inconjunction with the automatic tracking apparatus of Dye, to guardagainst the transmission drop out that would otherwise greatly degradethe nighttime usefulness of such apparatus.

In actual nighttime operation, the present invention might be employedin conjunction with an acquisition or "hailing" system such as thatdescribed in "GaAs Laser Communicator Acquisition System" by R. A. Dyeand C. R. Berry (Santa Barbara Research Center Information Paper, March1974) which determines the absence or presence of transmitting activityin the sector of interest. The system described in the above paperincludes, inter alia, apparatus for performing scanning and filteringfunctions in conjunction with a "maximum likelihood" detection schemewhereby the presence of an active (transmitting) communicator in thesector of interest can be determined. Once such a determination has beenmade, the listener need only scan the horizon while observing theillumination pattern of the LED's of the monocular in accordance withthe teachings herein to obtain (and maintain) a good (full reception)line-of-sight with the transmitting communicator.

Although the automatic self-alignment apparatus disclosed in the pendingpatent application of Dye provides "fine tuning" for the communicatorsystem, it is evident that the transceiver 28 might be mountedstationary with respect to the outer cylinder 26. In this manner, thehuman operator may perform the entire tracking function, simplyadjusting the orientation of the outer cylinder 26 as dictated by theLED portion. (It may be noted that the examples illustrated in FIGS. 4and 5 did not employ automatic tracking to aid the realignment oftransceivers.)

What is claimed is:
 1. A hand-held communication unit adapted to receiveencoded optical signals transmitted from a remote source, said unitcomprising:detector means for receiving the optical signal andconverting it into humanly perceptable information; focusing means forfocusing the optical signal from the source onto portions of thedetector; said detector being further adapted to generate locationsignals related to the point of incidence of the optical signal on saiddetector; a housing for the unit in which said focusing means anddetector means are located, said housing including handle means enablinga person to point the unit in the general direction of the remotesource; sighting means mounted to the housing through which the userlooks in an attempt to locate the source; and indicator means locatedwithin the sighting means and coupled to the location signal from thedetector, adapted to provide the user with a visual indication of thedegree of alignment of the communication unit with the source wherebythe user can more accurately point the housing towards the source tomaintain better communication therewith especially under low visibilityconditions.
 2. The unit of claim 1 wherein said detector is divided intoa plurality of different portions having a given geometry; andwhereinsaid sighting means includes a plurality of electroluminescent devicesarranged in a substantially similar pattern which are illuminated inresponse to incidence of said optical signal upon correspondinglyspatially located detector portions.
 3. The unit of claim 2 wherein saidsighting means includes at least one lens; andwherein saidelectroluminescent devices are located within the field of view of saidlens.
 4. The unit of claim 3 wherein the sighting means is a monocularhaving an eyepiece with a reticle thereon defining a plurality ofsegments generally corresponding to the geometry of the detectorportions, and wherein there is one electroluminescent device locatedwithin each eyepiece segment.